1. Field of the Invention
The present invention relates to audio-frequency power amplifiers and, more particularly, to an audio-frequency power amplifier that utilizes a bridged amplifier configuration.
2. Description of the Related Art
An audio-frequency power amplifier is a device that delivers power to a load, such as a speaker. Audio-frequency power amplifiers are commonly implemented in a number of ways. The efficiency by which these power amplifiers deliver power to the load, which is measured as the ratio of the power delivered to the load divided by the total power input to the amplifier, varies greatly among the different implementations.
One type of implementation, known as a Class A amplifier, is the most inefficient at delivering power to a load. Class A amplifiers have a linear region of operation, and are biased to operate from the center of the linear region of operation. When the input signal has no amplitude, a current that corresponds with the center of the linear region flows through the amplifier. When the amplitude of the input signal increases, the current increases, and when the amplitude decreases, the current decreases. As a result, a Class A amplifier consumes power throughout the entire cycle of the input signal, i.e., consumes power regardless of the amplitude of the input signal.
One example of a Class A amplifier is a power field-effect transistor (FET). A power FET has a source, a drain, a gate, and a linear region of operation. When configured as a Class A amplifier, a bias voltage is applied to the gate which, in turn, causes a current to flow through the FET which corresponds with the center of the linear region of operation.
When the input signal makes a positive excursion, the voltage on the gate is increased which causes the magnitude of the current to increase. When the input signal makes a negative excursion, the voltage on the gate is decreased which causes the magnitude of the current to decrease. Although inefficient from a power standpoint, Class A amplifiers provide a minimum amount of waveform distortion, and are thus widely used in audio systems.
Another implementation, known as a Class B amplifier, is more efficient at delivering power to a load than a Class A amplifier, but adds significantly more distortion to the output signal than a Class A amplifier. Class B amplifiers have a linear region of operation that ideally passes through the turn off point of the amplifier, and are biased to operate from the turn off point. Thus, when the input signal has no amplitude, no current flows through the amplifier. When the amplitude of the input signal increases, the current increases, and when the amplitude decreases, the current decreases.
One example of a Class B amplifier is an n-channel power FET connected in series with a p-channel power FET where the gates of the FETs form the input and the sources of the FETS form the output. When the input signal is equal to zero, both FETs are turned off. When the input signal makes a positive excursion, the n-channel FET turns on to source a current while the p-channel turns off. On the other hand, when the input signal makes a negative excursion, the p-channel FET turns on to sink a current while the n-channel turns off. Thus, since neither of the FETs in on when the input signal has no amplitude, and only one of the FETs is on when a positive or negative amplitude is present, a Class B amplifier is more efficient than a Class A amplifier.
As noted above, a Class B amplifier adds significantly more distortion to the signal than a Class A amplifier. This result occurs because power FETs do not have a linear region of operation that extends down to the turn off point. Instead, power FETs have a small non-linear region that lies between the turn off point and the linear region of operation. Thus, each time the input signal transitions by the turn off point, the signal is distorted.
Another amplifier, known as a Class A/B, is more efficient at delivering power to a load than a Class A (although less efficient than a Class B), and adds less distortion than a Class B (although more distortion than a Class A). Amplifiers that are categorized as Class A/B amplifiers utilize features taken from both Class A amplifiers and Class B amplifiers.
The above described example of a Class B amplifier can be converted into a Class A/B amplifier by applying a positive bias voltage to the gate of the n-channel FET and a negative bias voltage to the gate of the p-channel FET. The positive bias voltage is sufficient to place the n-channel FET at lower end of the linear region of operation, while the negative bias voltage is sufficient to place the p-channel FET at the upper end of the linear region of operation.
In this configuration, when the input signal has no amplitude, the n-channel sources a current which is sunk by the p-channel transistor. As the amplitude of the input signal increases, the n-channel FET linearly increases the current being sourced, while the p-channel non-linearly decreases the current being sunk, and then stops. Thus, the effect of the non-linear region of the p-channel FET is reduced by the stronger effect of the linear region of the n-channel FET.
Similarly, as the amplitude of the input signal decreases, the p-channel FET linearly increases the current being sunk, while the n-channel non-linearly decreases the current being sourced, and then stops. Thus, the effect of the non-linear region of the n-channel FET is reduced by the stronger effect of the linear region of the p-channel FET.
In this example, the Class A/B amplifier is less efficient than a Class B in that the n-channel FET is turned on during the positive excursions of the input signal and a portion of the negative excursions. In the Class B example, the n-channel FET was not turned on at all during the negative excursions of the input signal. Similarly, the p-channel FET is turned on during the negative excursions of the input signal and a portion of the positive excursions. In the Class B example, the p-channel FET was not turned on at all during the positive excursions of the input signal.
A Class A/B amplifier can also be implemented differentially as a bridged amplifier. FIG. 1 shows a schematic diagram that illustrates a conventional bridged amplifier 100. As shown in FIG. 1, amplifier 100 has a first operational amplifier (op amp) 110. First op amp 110, in turn, has a positive input connected to an input node N.sub.IN, and a negative input connected to a first intermediate node N.sub.1. In addition, op amp 110 also has an output connected to a first output node N.sub.out1. further, op amp 110 is connected to an upper supply rail VCC, and a lower supply rail VEE.
As additionally shown in FIG. 1, amplifier 100 also has a second operational amplifier (op amp) 112. Second op amp 112 has a positive input connected to a reference voltage V.sub.REF, and a negative input connected to a second intermediate node N2. In addition, op amp 112 also has an output connected to a second output node N.sub.OUT2. Further, op amp 112 is connected to the upper supply rail VCC, and the lower supply rail VEE.
Amplifier 100 additionally has a pair of feedback resistors RF1 and RF2, and a pair of input resistors RIN1 and RIN2. Feedback resistor RF1 is connected between the output and the negative input of op amp 110, while feedback resistor RF2 is connected between the output and the negative input of op amp 112.
Further, input resistor RIN1 is connected between the negative input of op amp 110 and the reference voltage V.sub.REF, while input resistor RIN2 is connected between the negative input of op amp 112 and the input node N.sub.IN. An input signal V.sub.IN can be directly applied to input node N.sub.IN if a common reference exists between the input voltage V.sub.IN and the reference voltage V.sub.REF, or can be applied via a capacitor C as shown in FIG. 1.
In operation, op amp 110 and resistors RF1 and RIN1 are connected together to form a non-inverting negative feedback circuit that outputs a first voltage V1 as described by equation 1 as: EQU V1=(1+(RF1/RIN1))(V.sub.IN -V.sub.REF)+V.sub.REF, EQ. 1
where the reference voltage V.sub.REF iS equal to the voltage that corresponds with the input signal having no amplitude, e.g., +2.5V for an input signal ranging from 0V to 5V.
Similarly, op amp 112 and resistors RF2 and RIN2 are connected together to form an inverting negative feedback circuit that outputs a second voltage V2 as described by equation 2 as: EQU V2=(-RF2/RIN2)(V.sub.IN -V.sub.REF)+V.sub.REF. EQ. 2
Thus, when the voltage on the input signal V.sub.IN is equal to zero, the voltages V1 and V2 are defined by the reference voltage V.sub.REF multiplied times a resistive multiplier: (1+(RF1/RIN1)) for voltage V1 and (-RF2/RIN2) for voltage V2. The expressions (1+(RF1/RIN1)) and (-RF2/RIN2) are multipliers that add voltage gain to the input signal V.sub.IN. For example, if resistors RF1 and RIN1 are both equal to 100.OMEGA., then the expression (1+(RF1/RIN1)) will always multiply the expression ((V.sub.IN -V.sub.REF)+V.sub.REF) by two.
Similarly, if resistor RF2 is equal to 200.OMEGA. and resistor RIN2 is equal to 100.OMEGA., then the expression (-RF2/RIN2) will always multiply the expression ((V.sub.IN -V.sub.REF)+V.sub.REF) by a negative two. As a result, the differential output voltage is two times the input voltage, e.g., a single-ended signal ranging from -2.5V to +2.5V is output differentially ranging from -5V to +5V.
An output version of the original input signal is formed as a difference between the first and second voltages, i.e., V1-V2. Thus, when the voltage on the input signal V.sub.IN is equal to zero, the difference V1-V2 is also equal to zero. When the input signal V.sub.IN makes a positive excursion, the voltage V1 output from op amp 110 increases while the voltage V2 output from op amp 112 decreases, thereby providing a larger differential voltage. On the other hand, when the input signal V.sub.IN makes a negative excursion, the voltage V1 decreases while the voltage V2 increases, thereby providing a smaller differential voltage.
One of the disadvantages of amplifier 100 is that, even though amplifier 100 receives a non-negative input signal, the lower supply rail VEE must be set to a negative voltage to accommodate the largest negative voltage output from op amps 110 and 112. For example, for an input signal ranging from zero-to-five volts, a +2.5V reference, and a resistor multiplier of two, the lower supply rail VEE must be set to -2.5V to accommodate the largest negative voltage of the first voltage V1 and the largest negative voltage of the second voltage V2. The requirement for a negative supply rail, however, adds additional cost and complexity.
Another disadvantage of amplifier 100 is that it is difficult to match the performance of op amps 110 and 112 such that the outputs of op amps 110 and 112 are clipped at the same time. Clipping occurs when the input voltage causes the output of an op amp to exceed the supply rail.
For example, assume that the upper supply rail VCC is set to +5V to accommodate the largest positive voltage output by the op amps, and the lower supply rail VEE is set to -5V to accommodate the largest negative voltage output by the op amps. In this case, an input voltage that would normally cause voltages V1 or V2 to exceed +5V is clipped because the voltages V1 and V2 cannot exceed the upper supply rail VCC of +5V. Similarly, an input voltage that would normally cause voltages V1 or V2 to fall below -5V is clipped because the voltages V1 and V2 cannot fall below the lower supply rail VEE of -5V.
Ideally, op amps 110 and 112 are matched such that an input voltage that just causes the first voltage V1 to be clipped also just causes the second voltage V2 to be clipped. In actual practice, however, variations in resistor values and other effects cause either the first voltage V1 to be clipped before the second voltage V2, or visa versa. The result of one voltage being clipped before the other causes the output voltage range of amplifier 100 to be reduced.
Another type of amplifier, known as a Class D, is very efficient at delivering power to a load. Class D amplifiers generate an amplified pulse train where the pulse widths of the pulses are modulated by the input signal. The pulse train is then fed into an inductor that averages out the pulse widths to recover an amplified version of the input signal.
One example of a Class D amplifier includes a comparator, and two series-connected switching devices which each have a gate connected to the output of the comparator. The comparator compares the voltage of the input signal with the voltage of a time varying reference signal, such as a sawtooth waveform.
In response to the comparison, the comparator outputs a pulse train. The pulses in the pulse train have pulse widths that are modulated by the results of the comparison. During each pulse period, when the signal is pulsed high, one of the switching devices is turned on while the other switching device is turned off, and visa versa when the signal is pulsed low. The action of the switching devices forms an amplified pulse train which has high and low values defined by the power supply rails. The amplified pulse train is then fed into an inductor that recovers an amplified version of the input signal.
The switching devices can be arranged so that there is very little voltage across a switching device at the time the switching device is carrying current, or so that there is a very high voltage across the switching device at the time the device is carrying very little current. Both of these are low power conditions.
Class D amplifiers, however, have a number of limitations that make them less than desirable for audio applications. One of the difficulties with Class D amplifiers is that the switching devices, which typically operate at very high frequencies, inject high frequency components into the signal.
These components then have to be filtered out before the signal is output to the load. Adding circuits to filter the signal, however, increases the cost and size of the amplifier. In addition, these high frequency components are also radiated from the inductor, thereby raising an electromagnetic interference (EMI) issue.
Another implementation, known as a Class G amplifier, is also very efficient at delivering power to the load. Class G amplifiers are connected to four power supplies: an upper supply V++ which is equal to the peak amplitude of the output signal, and a first mid supply V+ which is less than the upper supply. The remaining supplies include a lower supply V-- which is equal to the minimum amplitude of the output signal, and a second mid supply V- which is greater than the lower supply.
When the amplitude of the output signal ranges between the first mid supply V+ and the second mid supply V-, the Class G amplifier draws power from the first and second mid supplies V+ and V-. For signal excursions that go beyond these limits, the Class G amplifier switches and draws power from the upper and lower supplies V++ and V--.
Studies have indicated that music signals, which have a statistical nature, have a significant amplitude distribution at levels which are well below one-half the peak value. Thus, by using lesser supply voltages V+ and V- during the extensive periods of time that the amplifier output is equal to or less than one-half of the peak value, the amount of power consumed by the amplifier is reduced, thereby increasing the efficiency of the amplifier.
Another amplifier, known as Class H, is similar to the Class G amplifiers. Like Class G, Class H amplifiers are connected to four power supplies: an upper supply V++, a first mid supply V+, a lower supply V--, and a second mid supply V-. Also like the Class G amplifiers, when the amplitude of the output signal ranges between the first mid supply V+ and the second mid supply V-, the Class H amplifier draws power from the first and second mid supplies V+ and V-.
However, for signal excursions that go beyond these limits, the Class H amplifier modulates the upper and lower voltage supplies (V++) and (V--) to track the signal such that the voltage available to drive the output voltage remains a fixed number of volts greater than the output voltage.
FIG. 2 shows a schematic diagram that illustrates a conventional Class H bipolar-supply amplifier 200. As shown in FIG. 2, amplifier 200 includes an op amp 210 which has two signal inputs, a signal output, a positive supply input, and a negative supply input. In addition, amplifier 200 includes a first tracking circuit 212 that is connected to the output and the positive supply input of op amp 210, and a second tracking circuit 214 that is connected to the output and the negative supply input of op amp 210.
First tracking circuit 212, in turn, includes a tracking transistor 220, a diode D, and a first bias circuit 222. Transistor 220 has a gate connected to the first bias circuit 222, a drain connected to an upper power supply V++, and a source connected to a mid power supply V+ via diode D, and to the positive supply input of op amp 210.
In operation, first bias circuit 222 adds a predefined number of volts to the voltage on the output of op amp 210, and places the voltage on the gate of transistor 220. In addition, a voltage equal to one diode drop less than the mid power supply V+ is placed on the source of transistor 220 and on the positive supply input of op amp 210. When the voltage on the gate of transistor 220 exceeds the mid power supply V+, transistor 220 turns on and linearly increase the voltage on the positive supply input of op amp 210 as the voltage on the output increases.
For example, assume the case where the upper supply is equal to ten volts, the mid power supply is equal to five volts, and the bias voltage is equal to two volts. In this case, when the voltage on the output is equal to two volts, the voltage on the gate of transistor 220 is equal to four volts and the voltage on the source is equal to one diode drop less than five volts. When the voltage on the output is equal to three volts, the voltage on the gate of transistor 220 is equal to five volts and the voltage on the source is equal to one diode drop less than five volts.
When the voltage on the output is equal to four volts, the voltage on the gate of transistor 220 is equal to six volts and the voltage on the source is equal to one diode drop less than six volts. Thus, the voltage supplied to the positive supply input is at least two volts (less a diode drop) greater than the voltage on the output (until the upper supply is hit). In addition, second tracking circuit 214 operates in an identical, but opposite, fashion.